535 5.7.8: authentication failed for an external address
Fastmail could not sign in to the outgoing server of an external address you send from, most often because its stored password has changed.
- Fastmail
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 535 5.7.8
- Provider
- Fastmail
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
535 5.7.8 Error: authentication failed: authentication failure
What it means
When you send from an external address through Fastmail with authenticated sending, Fastmail signs in to that provider SMTP server on your behalf. This 535 5.7.8 means the sign-in failed. Fastmail says the likely cause is that the password it holds for that server needs updating, which happens after a password change at the other provider. Fix it in Settings, then My email addresses: open the address, choose Use authenticated SMTP when sending, and check the server settings and the password.
Common causes
- The password at the external provider changed.
- The server settings stored for the address are wrong.
How to fix it
- 01Open Settings, then My email addresses, and click the address you sent from.
- 02Click Use authenticated SMTP when sending.
- 03Use Check server settings, then Update password with the current password.
- 04Send the message again.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Why messages bounce back (Fastmail help center)fastmail.help
Related codes
- 551 5.7.1Not authorised to send from this address.Fastmail refused to send your message because you are not set up to send from the address in its From header. Verify the address first.
- 535SMTP authentication failed.Your email program could not sign in to the IONOS outgoing server because the username or password stored in it is wrong.
- 535 5.7.8Login failed: invalid credentials.The SMTP server rejected the login because the username, password or token was wrong or not sufficient, so it will not accept mail from this session.
5.7.8 at other providers
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